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<script>
  function arr(nums) {
    const n = nums.length;
    if (n <= 2) {
      return n;
    }
    let slow = 2, fast = 2;
    while (fast < n) {
      if (nums[slow - 2] != nums[fast]) {
        nums[slow] = nums[fast];
        ++slow;
      }
      ++fast;
    }
    return slow;
  };
  console.log(arr(nums = [1, 1, 1, 2, 2, 3]));
</script>

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